Alloys of the Al—Mg—Si type are preferentially used in diecasting processes, and they are especially advantageous for producing thin-walled components.
For example, for an Al—Mg—Si alloy of the following general composition: 5.0-6.0 weight % Mg, 1.8-2.6 weight % Si, 0.5-0.8 weight % Mn, and Al as the remaining ingredient, the breaking elongation [A5] is 16% for components with a wall thickness of 4 mm, 7% with a wall thickness of 18 mm, and only 4% with a wall thickness of 24 mm. Thus in workpieces produced by diecasting, a pronounced worsening of the breaking elongation with increasing wall thickness is found.
Furthermore, it is known that workpieces of alloys of the Al—Mg—Si type that have been produced by permanent-mold casting or sandcasting have poor mechanical properties, in particular with regard to their breaking elongation.
For instance, if an alloy having the following general composition: 4.5-6.5 weight % Mg, 1.5 weight % Si, 0.45 weight % Mn, and Al as the remaining ingredient, is used in permanent-mold casting or sandcasting, the breaking elongation [A5] is 3% for a workpiece with a wall thickness of 20 mm produced by sandcasting, and also 3% for a workpiece with a wall thickness of 16 mm produced by permanent-mold casting. Thus comparably poor breaking elongation values to those in diecasting are obtained.
To improve the mechanical properties of components, among other things particle refining treatments can be performed.
In diecasting, a particle refining treatment is generally unnecessary and can even have adverse effects. The solidification conditions in diecasting, and especially the high cooling rate, already effectively counteract particle growth. However, in the prior art, a treatment with melt treatment salts that contain halogen, such as MgCl2, or so-called, active gases, such as chlorine gas with nitrogen or argon, in various concentrations are known for achieving a fine microstructure and thus good mechanical properties.
It is moreover known that the microstructure of Al—Mg—Si alloys, in particular for diecasting, can be controlled by adding alloy elements such as Mn, Cr and Zr; see ASM Specialty Handbook: Aluminum and Aluminum Alloys, 1993, ASM International, p. 44.
From all the data sheets for the applicable alloys and in the literature, it can be learned that any addition of phosphorus, whether intentional or not, is to be avoided, since it counteracts an advantageous microstructural development and thus makes the mechanical properties of workpieces of these alloys worse.
Conversely, what is known in the prior art is an addition of phosphorus to Al—Si—Mg alloys; see for example ASM Specialty Handbook Aluminum and Aluminum. Alloys, 1993, ASM International. p. 44. The term Al—Si—Mg, in contrast to Al—Mg—Si, means that such an alloy has a higher proportion of Si than Mg.
The addition of phosphorus is done especially in near-eutectic and supereutectic Al—Si—Mg alloys. Supereutectic Al—Si—Mg alloys are alloys with an Si content that is slightly, or considerably, more than 12% Si. With a content of 12% Si. the alloy is exclusively eutectic, in the form of a fine-grained Al—Si mixed crystal.
In supereutectic Al—Si—Mg alloys, as the alloy melt cools, coarse-grained Si crystals develop first, which are subsequently embedded in the fine-grained mixed crystal structure. Because of the coarse Si crystals, the mechanical properties become worse. Adding AlP makes these Si crystals finer, since AlP acts as a nucleating agent for Si crystals, which are therefore present in markedly smaller sizes in the resultant structure, thereby improving the mechanical properties.
Conversely, adding phosphorus to subeutectic Al—Si—Mg alloys is ineffective, since as these alloys cool, α Al crystals form first, not Si crystals, and the eutectic Al—Si alloy forms after that.